I have implemented the ability to use rules very similar to column coloring rules to put icons in cells on calibre's spreadsheet. The rule can choose whether or not to leave the text in the cell. For example, you can put a "star" image next to any title that has a tag "favorite".

You will be able to use this new feature to create columns that are only icons. For example, you will be tell calibre to use some image in your column if the text is "!", and another image if the text is something else.

You need to give calibre an image file to use. You can get image files by searching for icons. Another way to create the images is to find/display something you like, do a "screen capture", and save the result as an image.

Just to play, I used the new feature to add some icons to the author column of a test library full of fake books. One rule adds a down-pointing triangle if the énum column contains a 5, an up-pointing triangle if the author contains an "i", and a "bug" if the author contains an "e". The attached image shows the result. The second attachment shows the rules.

Chaley I was looking at your samples above. Will one of the rules be "does not contain"? That option will be extremely useful.

The conditions available are exactly those available for column coloring. They depend on the type of the column the condition is based upon. For text columns (not like tags), the choices are "is", "is not", "matches pattern", "does not match pattern", "is set", and "is not set".

Try playing with coloring, using different colors to represent different icons. If you able to set the color of the text in a column to the "right" color then you will be able to set the icon for that text to the "right" icon.

After the update, on clicking the Add Rule button on the column icons tab, this error occurs:
calibre, version 0.9.17
ERROR: Unhandled exception: <b>WindowsError</b>:[Error 3] The system cannot find the path specified: u'C:\\Users\\****\\AppData\\Roaming\\calibre\\cc_i cons\\*.*'

Btw. I got the same error that kevinrs got. I just created the folder manually and it worked great.
I also wanted to tell you that if people are looking for icons this is a great site <http://www.iconfinder.com/>

I do have one question. If there is a column that is a date and you want to say <is less than> <today>. The column is looking for a date, how do I tell it today, whatever today is.